Kittens are always in higher demand than adult cats. You can often (but not always!) find them available for 'adoption', but those 'adoptions' often come with hefty 'adoption fees' that basically amount to a sale anyway ... especially for young kittens, again because of the higher demand.

Sometimes those 'adoption fees' are justified because it's a way the shelter funds food and medical care for that kitten and other animals in their care. Paying a $50 adoption fee for a kitten means the shelter has $50 to spend on taking care of the next animal in need. Not just food and shelter, but also vaccinations and spay/neuter operations. Other times, though, it's just a more morally acceptable way of selling animals ... especially used on platforms that ban sale of animals but allow adoptions with 'adoption fees' -- tons of that on Craigslist, where you're not allowed to sell pets, but you are allowed to list pets for adoption, including whatever 'adoption fee' you want to ask for.